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Re: Schematic for the NKMa psu + KM56 microphone
Author: Martin Davies
Date: 08/08/2017
Hi Uwe
Thanks very much.
I have those schematics and have read those discussions recently thanks to Klaus Heyne.
I thought his advice to use the mic and the tube until the tube is no longer useable was very sensible and pragmatic.
Otherwise I will need to build a second power supply (or retube both my KM64 mics and mod my dual power supply to use EC71 with 6.3V heater supply which seems like the wrong approach).
So it seems that if/when I need to replace the tube I can either buy a $500-600 AC701 which may be of an unknown quality and potentially sound the same as an EC71 or I can modify the mic and try my luck with EC71 and a second power supply.
And the audiokarma link gives very useful specifics on what modifications would need to happen to use an EC71 although the example they use is for a 7586 Nuvistor.
Essentially the steps would be the same. Test the power supply mod with a 2W 63ohm resistor as a substitute load for the tube initially and so on.
Are there any pitfalls that I would need to be aware of with the modification from AC701k to EC71?
Is there any truth in what I was told by a friend that a fingerprint on the 250MOhm resistor could have been the cause of the noise and that the circuit should be washed with isopropyl, water and isopropyl again?
Examples of the mic noise are here if you haven't heard them.
https://drive.google.com/open?id=0ByMXqT JH1cS4ZTZPakN3ZnBudHM
https://drive.google.com/open?id=0ByMXqT JH1cS4ejRuUWotOUFCMW8
Thanks
Martin  
 
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